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A comparative study on textural characterization: cation-exchange and sorption properties of crystalline α-zirconium(IV), tin(IV), and titanium(IV) phosphates
Journal of Colloid and Interface Science 270 (2004) 436–445 www.elsevier.com/locate/jcis
A comparative study on textural characterization: cation-exchange and sorption properties of crystalline α-zirconium(IV), tin(IV), and titanium(IV) phosphates K.M. Parida,∗ B.B. Sahu, and D.P. Das Regional Research Laboratory (CSIR), Bhubaneswar 751013, Orissa, India Received 20 May 2003; accepted 30 September 2003
Abstract Tetravalent metal phosphates (M = Zr, Ti, and Sn) were prepared and characterized by XRD, surface properties, and TG-DTA. The cation exchange and sorption behavior of these metal phosphates toward transition metal ions such as Cu2+ , Co2+ , and Ni2+ have been studied comparatively as a function of temperature and concentration. The adsorption process was found to increases with increase in temperature and concentration. The selectivity order for α-titanium and α-tin phosphates is Cu2+ > Co2+ > Ni2+ , whereas for α-zirconium phosphate it is Cu2+ > Ni2+ > Co2+ . The ion exchange capacity of α-titanium phosphate is greater than those of other phosphates, which is explained on the basis of the surface behavior, disorderness of the system, degree of hydrolysis of incoming guest adsorbate metal ions, and structural steric hindrance of the exchangers during adsorption and sorption. The distribution coefficient, Gibbs free energy, enthalpy, and entropy values indicate that the ion-exchange processes are spontaneous. 2003 Elsevier Inc. All rights reserved. Keywords: Tetravalent metal phosphates; Fibrous layered ion exchangers; Sorption behavior; Distribution coefficient; Gibbs free energy
1. Introduction An active search for new selective ion exchangers and adsorbents undertaken in the past several decades resulted in discovery of a wide variety of synthetic inorganic compounds exhibiting ion-exchange properties [1]. Because of their high thermal/chemical stability, resistance to oxidation and radiation, and greater affinity to certain ions, these compounds (hydrated oxides and insoluble acid salts of polyvalent metals, heteropolyacids, metal ferrocyanides, etc.) are regarded as promising materials for operating under extreme conditions such as high temperature, radiation, pressure, concentration of background electrolytes, and presence of oxidants and organic solvents [2]. The structure of the defined crystalline tetravalent metal phosphates is layered and exhibits ion-sieve and enhanced catalytic properties [3]. Besides possible application of these materials in chemical processing of radioactive materials, * Corresponding author.
E-mail address: [email protected] (K.M. Parida). 0021-9797/$ – see front matter 2003 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2003.09.045
treatment of contaminated moderator and cooling water in nuclear fields, new applications have recently been found in water desalination (hydrometallurgy) and in fuel cells employing ion-exchange membranes for transport of hydrogen ions and thin-layer chromatographic separation of Na+ from K+ (electrodialysis). Ion exchange is now a wellestablished technique in many industrial processes and is also widely employed in many chemical laboratories. Due to the presence of both Brønsted and Lewis acid sites on their surfaces, they have 102 –103 times greater ion-exchange capacity than commercial organic ion exchangers such as Dowex 50-X8. New developments in this field led to the synthesis of several exchangers having a fixed composition and a well-defined crystalline structure [4,5]. In the present work we have made a comparative study of the textural characterization and cation exchange cum sorption properties of α-zirconium, titanium, and tin phosphates toward alkali and divalent metal cations. The corrected selectivity coefficients of various parameters as a function of temperature, concentration, and metal loading were also analyzed.
K.M. Parida et al. / Journal of Colloid and Interface Science 270 (2004) 436–445
2. Experimental 2.1. Method of preparation and chemical analysis Crystalline zirconium(IV) phosphate [Zr(HPO4)2 ·H2 O, α-ZrP], tin(IV)phosphate [Sn(HPO4 )2 ·H2 O, α-SnP], and titanium(IV) phosphate [Ti(HPO4 )2 ·H2 O, α-TiP] were prepared and analyzed using the previous experimental procedures [6–8]. 2.2. Physicochemical characterizations X-ray diffraction studies of the samples calcined at 500 ◦ C were carried out using a Phillips PW 1710 diffractometer fitted with an automatic control and data acquisition system (PC-APD Software). The patterns were run with monochromatic CoKα radiation (α = 1.72 Å). The 2θ diffraction angle (3–70◦ ) was scanned at the rate of 2.4◦ min−1 . TG-DTA analysis of room temperature and 110 ◦ C dried samples were carried out in dry air using a Shimadzu DT 40 thermal analyzer in the range from 25 to 1000 ◦ C at a heating rate of 10 ◦ C min−1 . Surface acidity and basicity were determined by the spectrophotometric method [9] on the basis of irreversible adsorption of organic bases such as pyridine (PY, pKa = 5.3), piperidine (PP, pKa = 11.1), morpholine (MR, pKa = 16.6), 2,6-dimethyl piperidine (DMP, pKa = 11.11) and acidic substrates such as acrylic acid (AR, pKa = 4.2) and phenol (PH, pKa = 9.9), respectively. One electron donor and one electron acceptor properties were determined on the basis of irreversible adsorption of phenothiazine (PNTZ, ionization energy, I.E. 7.13 eV) and 1,3-dinitrobenzene (DNB, electron affinity, E.A. 1.26 eV). Potentiometric titration of materials in 0.1 mol/L (Na/K)Cl solution at different temperatures (300, 310, and 320 K) was carried out in a thermostatic double-wall Pyrex cell of capacity 100 ml with a rubber lid equipped with holes for the electrode and a microburette. In each of the experiments, 25 ml of electrolyte (Na/K)Cl and 0.1 g of exchanger were put into the cell with regular stirring until the pH become constant. After equilibrium, the pH of the suspension was measured by an Elico digital pH meter (L1 120, India) and then standard 0.01 mol/L (Na/K)OH solution was added from the microburette. The pH of the suspension was recorded when the pH drift was less than 0.01 unit/min. The addition of base was continued until the pH of the solution reached 9.0. The adsorption of first-row transition metal ions (Cu2+ , 2+ Ni , and Co2+ ) was carried out with 50 ml of the metal ion solution in a 100 ml stoppered flask. About 0.1 g of metal phosphate was added to the solution and the pH was measured. The resulting suspension was shaken mechanically with a shaker (Julabo) for 24 h. The solution was filtered and the equilibrium pH of the filtrate was measured. Preliminary experiments showed that 24, 30, and 48 h are required
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to reach the equilibrium pH for α-SnP, α-ZrP, and α-TiP, respectively. The pH was maintained in the range 2.7–4.1 for α-ZrP, 2.8–4.2 for α-SnP, and 2.6–3.8 for α-TiP. The concentration of the metal ion in the filtrate was determined with an atomic absorption spectrophotometer (Perkin–Elmer). The adsorption experiments were repeated at four different concentrations (8, 6, 4, and 2) ×10−4 mol l−1 of the metal ions.
3. Results and discussion 3.1. Textural properties The powder XRD pattern of 383 K heated α-ZrP, αTiP, and α-SnP showed crystalline structures having basal spacings 7.56, 7.6, and 7.76 Å, respectively, which is quite similar to the data reported earlier [10–12]. The Na+ and K+ exchanged form of tetravalent metal phosphates seems to exhibit a higher degree of crystallinity. From the powdered XRD pattern of α-ZrP, α-TiP, and α-SnP calcined at 773 K (Fig. 1), it was observed that the zirconium phosphate is completely converted into zirconium pyrophosphate (ZrP2 O7 ) and the case of tin phosphate is similar to that of zirconium phosphate. But in the case of titanium phosphate, at 773 K it is partly converted into pyrophosphate. From the thermogravimetric analysis (graph not given), it is observed that the total weight loss of α-ZrP, α-TiP, and α-SnP at 1173 K is 14.6%, 14.2%, and 14.1% corresponding to loss of 2.44, 2.03, and 2.22 moles of water per mole
Fig. 1. The XRD powder pattern of 773 K calcined (a) α-ZrP, (b) α-TiP, and (c) α-SnP.
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of material, respectively. The DTA curve exhibited two endothermic peaks at 468 and 531 K for α-ZrP, 573 and 773 K for α-TiP. However, there is only one endothermic peak at 429 K for α-SnP. The exothermic peak at 843 K for α-ZrP indicated a phase transformation of this material from the layered to a nonlayered structure corresponding to the formation of α-ZrP2 O7 . An exothermic peak for α-TiP was detected at 1086 K, which represents the transformation of α-TiP to α-TiP2 O7 [13]. Since one mole of H2 O must be related to the phosphate condensation, there could be two possibilities for the dehydration of α-SnP, which can be represented as follows: SnO(H2PO4 )2 Sn(HPO4 )2 + H2 O↑ α-Sn(HPO4 )2 ·H2 O → α-SnP2 O7 + 2H2 O↑. However, in contrast to other crystalline exchanges of this series, the weight loss curve does not distinctly show the two dehydration steps, and this fact could be explained on the basis of the overlapping of the two processes. To corroborate this hypothesis, the pyrophosphate content of this sample, heated at 623 K, indicated that 31% of the phosphate groups were converted to pyrophosphate. It is confirmed that the condensation begins before the first water molecule is completely lost. Further it was found that the α-SnP sample heated at 623 K and then suspended in water for 5 days takes up only 0.65 mol of water per formula weight, which is in good agreement with the value calculated using the pyrophosphate content. The monolayer adsorption of all the materials was assumed to be the acidity or basicity of the samples corresponding to a specific pKa of organic substrate used. The adsorption of organic substrates follow the Langmuir adsorption isotherm. From the adsorption data it has been observed that the chemisorption of piperidine and of acrylic acid show highest values of acid sites and basic sites, respectively, for all the samples. The general sequence obtained in all the cases for acid sites is PP > MR > PY > DMP. Ta-
ble 1 shows that the concentration of acid and basic sites of the materials increases with an increase in calcination temperature from 383 to 573 K and thereafter decreases to a low value at 773 K. It is well known that addition of transition metal to the metal phosphates enhances the concentration of one electron acceptor (oxidizing) site and one electron donor (reducing) site on the surface. Among all samples, α-TiP showed maximum increase in acid, basic, oxidizing, and reducing sites as compared to other materials. The oxidizing and reducing properties determined by the adsorption of DNB (m-dinitrobenzene) and PNTZ (phenothiazine), respectively, showed very little increase in these values with increase in calcination temperature up to 573 K, but decreased to a very low value beyond 773 K. As a consequence of the loss of bonded –OH groups above 573 K the above surface chemical properties of all materials decreased to a low value at 773 K. The above surface chemical properties decreased to a low value due to the loss of Brønsted acid sites. 3.2. Potentiometric titration The potentiometric titration curves of α-ZrP, α-TiP, and α-SnP in 0.1 mol/L of (Na/K)Cl at 300 K are presented in Fig. 2. For each case with addition of 0.01 mol/L of (Na/K)OH the pH increases. For α-ZrP, it exhibits two plateaus corresponding to the replacement of two hydrogens per mole of α-ZrP. The isotherm for α-ZrP shows two inflections below 7.5 and afterward it appears to be complicated due to hydrolysis effects. From Fig. 2 it was found that TiP is less hydrolyzed than ZrP and SnP. In the case of α-SnP
Table 1 Surface acidity, basicity, one-electron-donor, and one-electron-acceptor properties of α-ZrP, α-TiP, and α-SnP calcined at various temperatures Sample Temperature (◦ C)
Acid sites (µmol g−1 ) PP
Basic sites (µmol g−1 )
MR PY DMP AR
Redox sites (µmol g−1 )
PH PNTZ DNB
α-TiP
110 300 500
325 310 161 357 332 173 318 282 83
82 105 63
166 136 11.30 178 162 12.62 160 109 9.86
8.82 9.96 8.63
α-ZrP
110 300 500
316 281 80 341 299 133 276 202 69
75 100 42
162 111 10.06 163 132 11.37 160 102 9.50
8.65 9.52 8.28
α-SnP
110 300 500
275 223 294 241 236 201
67 90 33
153 119 10.01 160 124 11.14 111 100 9.20
8.48 9.42 8.10
94 96 87
Fig. 2. Potentiometric titration of α-ZrP, α-TiP, and α-SnP in an aqueous solution of 0.1 mol l−1 MCl (M = K and Na) at room temperature.
K.M. Parida et al. / Journal of Colloid and Interface Science 270 (2004) 436–445
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Fig. 3. Potentiometric titration of α-TiP in an aqueous solution of 0.1 mol l−1 KCl in the presence of 50 ppm Cu2+ , Co2+ , and Ni2+ solution at 300 K.
Fig. 4. Potentiometric titration of α-SnP in an aqueous solution of 0.1 mol l−1 KCl at different temperatures.
there is a series of irregular kinks. There is no end point and no break point. This is an indication of weak ion exchanger behavior. From Fig. 3, it can be seen that the crystalline α-TiP shows a sigmoidal curve, which means that the surface selectivity of the exchanger toward various cations decreases as they progressively replace the protons from the surface of the microcrystals. Up to pH 4.0 there is negligible hydrolysis and the result differs slightly in the pH range 4.0–7.0 for α-TiP. The layered crystal structure of [Ti (HPO4 )2 ·nH2 O] is built up by bonding together these sandwiches by van der Waals forces. In this arrangement each P atom in the lower sandwich lies along a perpendicular line drawn from the Ti atom of the upper sandwich. This arrangement creates zeolite type cavities that are interconnected by windows whose maximum diameter is around 2.64 Å. Both H-atoms can be exchanged with other cations so that the total ion exchange capacity is 7.76 meq g−1 [14], which is greater than for α-ZrP (6.64 meq g−1 ) and α-SnP (6.08 meq g−1 ) and 4–12 times those of clay minerals [15]. At pH 7, the Na+ uptake on α-TiP is 7.15 meq g−1 . At higher pH values increased hydrolysis is observed and the ion-exchange capacity decreases. This results in a higher degree of conversion at higher pH and a decrease in ion exchange capacity caused by hydrolysis. Potentiometric titration curves of α-SnP at different temperature (300, 310, and 320 K) are presented in Fig. 4. It indicates that the pH of the suspension increases with the addition of 0.01 mol/L of KOH, which is in contrast to other systems [13] where the pH remains constant with the ion uptake.
Interestingly, for all the exchangers, the curve shows that the pH of the solution shifts toward lower values with increased temperature. This indicates that H+ liberation is facilitated by an increase in temperature, which can be expressed in the reaction x−n nRH + Mx+ + nH+ (aq) → Rn M (aq)
(1)
where RH is the exchanger and Mx+ is the metal ion. This shows that, with an increase in temperature, more and more H+ ions are displaced by the K+ /Na+ ions. Hence sorption of the metal ions varies proportionally with temperature. It was found that for α-TiP, the prolonged branch of the titration curve above pH 9.2 (300 K), 9.0 (310 K), and 7.8 (320 K) may be explained by titration of less acid groups including ≡Ti–OH groups [16]. Fig. 3 shows that all the titration curves of α-TiP in 0.1 mol l−1 KCl with different metal ions are placed at a lower pH value as compared to the blank titration, which was also observed in case of α-SnP [7], α-ZrP [11], AlPO4 , and Al(OH)3 [12]. Thus the mechanism of metal ion uptake in this case is similar to that of adsorption on oxides/hydroxides, which can be described by Eq. (1). The number of replaceable protons responsible for the ion exchange reaction can be estimated from the difference between the blank run and each titration curve in the presence of the exchangers. Thus the excess amount of base consumed in the presence of an exchanger gives the extent of sorption of metal cations. Therefore Eq. (1) can be rewritten by applying the law of mass action as K = (R)n Mx−n [H+ ]n [RH]n [Mx+ ]
(2)
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where K is the equilibrium constant and the symbols in the square brackets represent the respective concentrations. In the region of low adsorption, [RH]n and [Mx+ ] can be treated as constants and [(Rn )Mx−n ] is equal to Cb /n, where Cb is the volume of 0.01 mol l−1 KOH/NaOH added. Thus Eq. (2) can be simplified to K = [Cb /n][H+ ]n
(3)
or log Cb = [log K]n + npH.
(4)
Using Eq. (4), log Cb vs pH can be plotted to determine log K and n (Fig. 5). Plots of log Cb as a function of pH for the potentiometric titration of K+ ions for α-SnP are presented in Fig. 5. The values of log K and n for the exchanger are presented in Table 2. Interestingly, the curves (Fig. 5) show two distinct lines, indicating the complex mechanism of K+ ion sorption on
Fig. 5. Plot of log Cb vs pH for 0.1 mol l−1 KCl solution at different temperatures.
Table 2 Values of log K and n calculated from Eq. (1) for K+ exchange on α-TiP, α-ZrP, and α-SnP Temperature (K)
log K
n
α-TiP
300 310 320
−7.59 −7.73 −7.98
1.00 1.03 1.05
α-ZrP
300 310 320
−7.60 −7.76 −7.94
1.00 1.01 1.03
α-SnP
300 310 320
−7.61 −7.78 −7.93
1.00 1.10 1.20
Sample
α-SnP. This complication in the plot clearly indicates the inadequacy of Eq. (4) with the simplifying assumption as well as the presence of different types of exchange sites on α-SnP. The result also shows that Eq. (4) is only applicable between pH 3.0 and 5.0, indicating the limitation of the simplification. Therefore the equation is more appropriate at a low adsorption density. The break points for all three lines are the same, which shows that the uptake of K+ ions is independent of temperature. This probably shows that the number of high-energy sites responsible for the uptake of K+ ions is independent of temperature (300–320 K). The value of n determined (Table 2) from the line within the low pH region is nearly equal to 1, which suggests the ion-exchange stoichiometry for alkali metal ions in the exchange as RH + M+ → RM + H+ .
(5)
The low value of n at 300 K point toward the mechanism of metal-ion uptake rather than ion exchange. Other mechanisms could be simple adsorption or partition of electrolyte between the phases. The zeolitic nature of the exchangers and the weak forces between layers provide a basis for explaining their sieving behavior and ready expansion of the lattice during ion exchange [10]. Both phosphate hydrogen ions (HPO4 ) are replaced by cations. One proton is exchanged at a relatively low pH for alkali metal cations and the interlayer distance remains unchanged. Thus at half the exchange capacity, all the cavities contain one univalent cation and can accommodate no more. The second proton exchanges at higher pH and the cations insert themselves between the layers. Thus the mechanism of exchange is thought to involve the diffusion of unhydrated or partially hydrated ions into the cavities, replacing the more acidic phosphate protons. This is followed by diffusion of water molecules into the crystal lattice and subsequent rehydration of the cations. Further exchange can then occur by diffusion of hydrated ions into the sorbent cavities [17]. The Na+ is exchanged as the hydrated ion and K+ as the unhydrated ion [18]. 3.3. Adsorption of heavy metal ions Adsorption studies on α-ZrP, α-TiP, and α-SnP, carried out using suspensions containing different concentrations of Cu2+ , Co2+ , and Ni2+ ions at different temperatures, are shown in Fig. 6. It can be seen that the metal phosphates take up these metal ions in appreciable quantities and the uptake generally increases with increased in temperature and concentration of the metal ions. Selectivity of these exchangers for transition metal ions is found in the order Cu2+ > Co2+ > Ni2+ for α-TiP and α-SnP, whereas Cu2+ > Ni2+ > Co2+ is for α-ZrP. The same selectivity sequence as for α-ZrP was noted with Fe(OH)3 [19], SiO2 [20], ThO2 [21], and TiO2 [7], and of α-SnP, α-TiP was noted with AlPO4 [22], FePO4 [23], SnO2 [24], and MnO2 [25]. The sequence order for α-TiP and α-SnP is
K.M. Parida et al. / Journal of Colloid and Interface Science 270 (2004) 436–445
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where RH2 is the exchanger and RM is the exchanged form, and + Sn(HPO4 )2 ·H2 O + Cu2+ (aq) → SnCu(PO4 )2 ·H2 O + 2H(aq) .
The adsorption data were fitted to linearly transformed Langmuir adsorption isotherms as Ceq /X = (1/bXm ) + Ceq /Xm ,
Fig. 6. Adsorption isotherm of Cu2+ , Co2+ , and Ni2+ on α-ZrP, α-TiP, and α-SnP at 300 K.
based on the stability of hexa-aqua ions, whereas for α-ZrP, this order can be related neither to the stability of hexa-aqua ions nor to the stereochemistry adopted by M2+ ions in the solid, which is distorted octahedral. However, comparing the extent of sorption of metal ions, the exchanger prefers the cation with low ionic potential pKa and low pH of hydrolysis (pKh ), where Kh is the hydrolysis constant of the metal cation in solution. As Cu2+ has the lowest pKh (=8.0), the adsorption is highest in comparison to Co2+ (pKh = 8.9) and Ni2+ (pKh = 9.9) [26]. The ion exchange and sorption properties increase with the increase in degree of crystallinity, which varies reciprocally with the interlayer spacing and surface area of the respective ion exchangers. Due to the minimum interlayer spacing and surface area of α-TiP as compared to α-ZrP and α-SnP, it has maximum ion exchange capacity (7.76 meq g−1 ). From the structures of these materials, it is seen that the Ti–Ti, Zr–Zr, and Sn–Sn distances are 5.3, 5.0 and 4.8 Å, respectively [27]. Therefore, the cavities of α-TiP are larger with a smaller steric hindrance for the incoming metal ions than others. As a result, sorption media facilitate its greater ion-exchange behavior. Due to the maximum interlayer distance of α-TiP, it requires a low activation energy (Ea ) for ion-exchange kinetics. In order to determine 2+ the mechanism of this process, H+ release /Madsorption for all the metal ions is given in Table 3. It can be concluded that on the average, 2 mol of H+ ions are released per 1 mol of metal ion adsorbed. This type of observation was also reported for the adsorption of various bivalent and trivalent cations on an oxide/hydroxide surface [19]. Therefore the exchange reaction can be expressed as RH2 + M2+ → RM + 2H+ ,
(6)
(7)
where Ceq is the equilibrium concentration of adsorbate in solution (mol l−1 ), X is the amount of adsorbate per unit mass of adsorbent (mol g−1 ), Xm is the amount adsorbed to form a monolayer (mol g−1 ), and b is the binding constant. Plots of Ceq /X vs Ceq give straight lines for α-ZrP, α-TiP, and α-SnP for all the metals Cu2+ , Co2+ , and Ni2+ (Figs. 7a, 7b, 7c, respectively) at 300 K with a correlation coefficient greater than 0.97 (Table 4), which favors the applicability of the Langmuir equation between pH 4.0 and 5.0. The values of Xm and b as determined from the slopes and intercepts of the lines at different temperatures are presented in Table 4. In order to obtain more quantitative information on the selectivity of exchangers, the distribution coefficient (Kd , ml g−1 ) values of the metal ions at different concentrations have been determined. The marked selectivity toward ions seen as Cu2+ , Co2+ , and Ni2+ can be pointed out so that the exchangers could be employed for several separations of analytical interest. According to A. Clearfield [27], Kd is calculated according to the formula 0 c c V /m − Ceq )/Ceq Kd = (Ceq (8) 0 and C c are the where Kd = distribution coefficient, Ceq eq ionic concentration of the initial solution and the solution after equilibrium with adsorbent, respectively. V /m (ml g−1 ) is the volume to mass ratio of the ion exchanger. According to Jorgen Albertsson [28]
log Kd = npH + D
(9)
where D = log K + n log(C − nCMG )
(10)
and 0 c CMG = (Ceq − Ceq )/(V /m) which is the total concentration of metal ion in the solution at equilibrium; C is the total exchange capacity of metal ion. When CMG is sufficiently small (CMG ≪ C), i.e., all sorption sites are equivalent, D is independent of CMG . Second, if the load (CMG ) on the ion exchanger is very low, and the constant value of CH (the hydrogen ion concentration) is so high that the solution can be considered to have a medium of constant ionic strength, it is found that Kd is a rectilinearly (rapidly) decreasing function of CMG alone, which indicates that sorption sites are not equivalent. It is observed from Table 5 that the sorption capacity of the solid
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Table 3 Stoichiometry of metal ion exchange on α-ZrP, α-TiP, and α-SnP at different concentrations at 320 K Sample α-TiP
α-ZrP
α-SnP
Metal ion
Initial metal ion conc. (10−4 mol l−1 )
Equilibrium conc. (10−4 mol l−1 )
Metal ion adsorbed, X (10−4 mol l−1 )
H+ released (10−4 mol l−1 )
H+ /M2+
Cu2+
2 4 6 8
0.003 0.063 0.150 0.263
1.997 3.94 5.85 7.737
3.99 8.27 11.99 16.01
2.00 2.10 2.05 2.07
Co2+
2 4 6 8
0.265 1.200 2.200 3.000
1.735 2.800 3.800 5.000
3.47 5.96 7.63 10.25
2.00 2.13 2.01 2.05
Ni2+
2 4 6 8
0.401 1.424 3.000 4.520
1.599 2.588 3.000 3.48
3.19 5.20 6.21 7.27
2.00 2.01 2.07 2.09
Cu2+
2 4 6 8
0.016 0.080 0.141 0.240
1.984 3.920 5.859 7.760
3.96 7.95 12.18 16.60
2.00 2.03 2.08 2.14
Co2+
2 4 6 8
0.488 1.640 3.180 4.612
1.512 2.360 2.820 3.388
3.05 4.79 5.86 7.04
2.02 2.03 2.08 2.07
Ni2+
2 4 6 8
0.300 1.230 2.220 3.120
1.700 2.770 3.780 4.880
3.36 5.56 7.52 9.81
1.98 2.01 1.99 2.01
Cu2+
2 4 6 8
0.020 0.085 0.154 0.266
1.980 3.915 5.846 7.734
3.97 7.83 11.10 14.46
2.01 2.00 1.90 1.87
Co2+
2 4 6 8
0.305 1.471 2.389 3.267
1.695 2.529 3.611 4.733
3.55 5.03 7.14 9.32
2.10 1.99 1.98 1.97
Ni2+
2 4 6 8
0.489 1.654 3.199 4.719
1.511 2.346 2.801 3.281
3.17 4.69 5.57 6.33
2.10 2.00 1.99 1.93
is very high and gradually decreases with decreased metal ion concentration. It is also seen that the adsorption sites are not of equal strength and the numbers of sites are different for different temperatures. Thus it is expected that the solid surface is covered by a monolayer of metal ions and a maximum number of sorption sites are active for metal ion adsorption. But beyond monolayer coverage, the metal ions repel the incoming homolog. The formation of monolayer coverage depends on the process temperature. From the variation of values of Kd (distribution coefficient) (Table 5) the sorption sites are not equivalent. That means that if the surface of the adsorbent is energetically heterogeneous rather than homogeneous, each step of the potentiometric titration isotherm will be replaced by an assortment of steps, corresponding to the completion of a monolayer on
different homogeneous patches of the surface when an ion exchange process occurs with a phase transition, giving rise to an ionic form having a larger interlayer distance than the original one, the formation of a metastable supersaturated solid solution is possible in the reverse process. As a result the sorption sites are not equivalent. Due to the greater hydrolysis constant of Co2+ than Ni2+ , the former should have a higher ionizing capacity. But evidently Ni(II) is sorbed more strongly than Co(II) for α-ZrP due to a greater separation factor [Kd (Ni)/Kd (Co)], varying between 1.56 to 2.13, whereas this factor is less than 1 for α-TiP and α-SnP (0.46 to 0.60 for α-TiP and 0.47 to 0.55 for α-SnP). The log Kd vs pH functions are straight and approximately parallel lines, which, however, do not coincide even
K.M. Parida et al. / Journal of Colloid and Interface Science 270 (2004) 436–445
(a)
443
(b)
(c) Fig. 7. Langmuir plots for the adsorption of (a) Cu2+ , (b) Ni2+ , and (c) Co2+ on α-ZrP, α-TiP, and α-SnP at 300 K.
at the lower loads investigated. The slope of the lines reaches a theoretical value of 2 for divalent cations. The apparent equilibrium constant (Ka ) (Table 6) corresponding to the adsorption process was calculated [20] as the product of the Langmuir parameters b and Xm and can be used as relative indicators of the affinity of the adsorbents toward the metal ions. The data (Table 2) shows that the affinity of α-ZrP, α-TiP, and α-SnP toward metal ions increases with an increase in temperature. From the plots of ln b vs T −1 (Fig. 8), the standard enthalpy (H 0 ) and en-
tropy (S 0 ) changes were calculated using the equation ln b = S 0 /R − H 0 /RT .
(11)
From the laws of thermodynamics (Gibbs–Helmholtz equation), G0 = H 0 − T S 0 .
(12)
By using values of S 0 and H 0 in Eq. (12), the Gibbs free energy (G0 ) can be calculated. All the values S 0 , H 0 , and G0 are presented in Table 7. The positive value of
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Table 4 Langmuir parameter Xm (mol g−1 ) and binding constant b for metal ion exchange on α-TiP, α-ZrP, and α-SnP Cu2+
Sample Temperature (K)
104 Xm
α-TiP
300 310 320
α-ZrP
α-SnP
Co2+ b
104 Xm
7.213 7.219 7.747
106266 114004 120601
300 310 320
6.120 6.234 6.618
300 310 320
5.004 5.062 6.514
Ni2+
Table 6 Apparent equilibrium constant (Ka ) and correlation coefficient calculated from Langmuir equation Ka
Sample Temperature
Correlation coefficient
(K)
Cu2+ Co2+ Ni2+
α-TiP
300 310 320
7.665 2.630 0.542 0.9924 0.9962 8.230 4.460 0.666 0.9864 0.9866 9.343 7.933 0.740 0.9821 0.9821
0.9998 0.9845 0.9716
47619 57871 58219
α-ZrP
300 310 320
6.205 0.423 2.000 0.9921 0.9721 6.310 0.565 2.463 0.9823 0.9723 7.216 0.629 3.028 0.9723 0.9746
0.9923 0.9810 0.9723
7958 13259 18315
α-SnP
300 310 320
5.267 0.234 0.197 0.9795 0.9864 5.595 0.503 0.250 0.9753 0.9874 8.149 0.523 0.349 0.9815 0.9819
0.9883 0.9997 0.9954
b
104 Xm
b
5.353 5.520 6.006
49131 80797 132084
0.908 0.970 0.973
59691 68659 76053
101388 101219 109035
1.790 1.960 2.001
23631 28826 31434
4.200 4.256 5.201
105267 110533 125109
2.946 3.594 3.613
11109 14019 14484
1.780 1.890 1.910
Cu2+
Co2+
Ni2+
Table 5 Distribution coefficient (Kd ) of different metal ion exchanges on α-ZrP, α-TiP, and α-SnP at different concentrations and temperatures Sample Metal ion α-ZrP
α-TiP
α-SnP
Initial metal
300 K
310 K
320 K
ion conc. log Kd pH log Kd pH log Kd pH (10−4 mol l−1 )
Cu2+
2 4 6 8
4.79 4.38 4.31 4.20
3.4 3.1 2.9 2.8
4.69 4.38 4.32 4.26
3.4 3.1 2.9 2.8
4.69 4.38 4.32 4.22
3.4 3.1 2.9 2.7
Co2+
2 4 6 8
3.19 2.85 2.64 2.56
3.5 3.3 3.2 3.1
3.18 2.86 2.64 2.57
3.5 3.3 3.2 3.1
3.18 2.86 2.66 2.59
3.5 3.3 3.2 3.1
Ni2+
2 4 6 8
3.45 3.05 2.92 2.89
3.5 3.3 3.1 3.0
3.45 3.06 2.93 2.89
3.4 3.2 3.1 3.0
3.48 3.09 2.94 2.91
3.5 3.2 3.1 2.9
Cu2+
2 4 6 8
5.52 4.49 4.29 4.16
3.3 3.1 2.9 2.8
4.99 4.69 4.51 4.38
3.4 3.1 3.0 2.8
5.00 4.70 4.56 4.39
3.5 3.2 3.0 2.9
Co2+
2 4 6 8
3.51 3.06 2.93 2.92
3.4 3.2 3.1 2.9
3.50 3.17 3.04 2.96
3.4 3.2 3.0 2.9
3.52 3.18 3.04 2.97
3.4 3.2 3.0 2.9
Fig. 8. Plot of ln b vs T −1 for different metal ions.
Ni2+
2 4 6 8
3.29 2.95 2.69 2.58
3.4 3.2 3.2 3.1
3.30 3.19 3.10 3.06
3.5 3.2 3.0 2.9
3.32 3.19 3.11 3.08
3.5 3.2 3.1 2.9
Table 7 Entropy, enthalpy, and Gibbs free energy changes of metal ions exchanged on α-TiP, α-ZrP, and α-SnP as a function of temperature
2 4 6 8
4.69 4.36 4.27 4.16
3.4 3.1 2.9 2.8
4.76 4.74 4.55 4.46
3.4 3.1 2.9 2.8
4.57 4.55 4.34 4.20
3.4 3.1 2.9 2.8
2 4 6 8
3.44 2.93 2.87 2.85
3.4 3.3 3.1 3.0
3.64 3.45 3.20 3.10
3.4 3.1 3.0 2.9
3.68 3.38 3.22 3.09
3.4 3.2 3.0 2.9
2 4 6 8
3.18 2.85 2.64 2.54
3.5 3.3 3.2 3.2
3.26 2.97 2.69 2.52
3.5 3.3 3.2 3.2
3.25 2.88 2.71 2.55
3.5 3.3 3.2 3.2
Cu2+
Co2+
Ni2+
Sample Metal ion
S 0 H 0 (JK−1 mol−1 ) (KJ mol−1 )
G0 (KJ mol−1 ) 300 K
310 K
320 K
Co2+ Ni2+
204.33 153.46 111.20
57.91 48.16 19.84
−61.26 −63.28 −65.33 −45.99 −47.53 −49.06 −29.73 −30.73 −31.71
α-ZrP
Cu2+ Co2+ Ni2+
165.00 101.25 118.23
41.26 18.00 34.50
−30.26 −31.26 −34.00 −24.64 −25.23 −27.47 −27.00 −28.68 −29.12
α-SnP
Cu2+ Co2+ Ni2+
119.42 113.51 100.58
10.17 7.03 6.85
−28.79 −29.98 −31.18 −23.87 −25.01 −26.15 −23.31 −24.32 −25.33
α-TiP
Cu2+
K.M. Parida et al. / Journal of Colloid and Interface Science 270 (2004) 436–445
H 0 indicates the endothermic nature of the ion-exchange system. The negative value of G0 favors the adsorption process. S 0 is positive for all the experiments, indicating an increased disorderedness in the system. In comparison with the ions of the same valency, the enthalpy change decreases with the atomic number of the metal ion, corresponding to a decrease in the entropy change of dehydration in response to an enthalpy–entropy compensation relation for α-ZrP only. This relation has been found in a wide variety of processes and reaction equilibria, heterogeneous catalysis, diffusion in metals, ionic crystals, and so on [29]. In the ion exchange system, the relation implies that the contribution of the electrostatic enthalpy term to the free energy is more or less fully compensated for by a simultaneous change of entropy term related to the hydration of the metal ion and the ion-exchanger phase. Thus the sorption of water molecules is responsible for the entropy change. The positive values of S 0 indicate the partial dehydration of the metal ions before adsorption, thus increasing the spontaneity. Again both H 0 and S 0 values followed the same order (Cu2+ > Co2+ > Ni2+ ) as the selectivity shown by α-TiP and α-SnP, whereas for α-ZrP the order is Cu2+ > Ni2+ > Co2+ . From these results, it can be concluded that changes in the hydration of metal ion play a dominant role in determining the selectivity of the exchanger. The highest disorder of the system was for Cu2+ (Table 7) as a result of hydration. This is also supported by the highest value of H 0 in the series, which indicates that a higher energy is needed to dehydrate Cu2+ ions than other mentioned ions. The value of G0 becomes more negative with increasing temperature, which shows that the adsorption is favored by an increase in temperature. The free energy changes responsible for the metal ion with H+ exchange as a function of the radius of the incoming cation show a departure from a linear relationship. This behavior is interpreted in terms of increasing distortion of the ion-exchanger matrix with the size of the incoming ion.
4. Conclusions From the foregoing discussion, it is clear that crystalline α-zirconium(IV), titanium(IV), and tin(IV) phosphate behave as acidic ion exchangers and have appreciable sorption capacity toward transition metal ions. The selectivity of metal ions for titanium(IV) and tin(IV) phosphates is in the order Cu2+ > Co2+ > Ni2+ , whereas for zirconium(IV) phosphate it is Cu2+ > Ni2+ > Co2+ . In all cases the extent of adsorption increases with increased temperature and concentration. Appearance of a plateau for α-ZrP, irregular kinks for α-SnP, and a sigmoidal curve for α-TiP in the potentiometric titration curves justify the presence of several exchangeable adsorption sites with different pKa values of
445
metal ions. The present investigation also shows that the ion exchange capacity (α-TiP > α-ZrP > α-SnP) depends on the degree of crystallinity, steric hindrance, sorption medium of exchangers, degree of hydrolysis, ionic potential, crystalline radius and charge of metal ions, and various thermodynamic parameters involved in the system.
Acknowledgments The authors are thankful to Dr. Vibhuti N. Misra, Director, Regional Research Laboratory, Bhubaneswar for his permission to publish this paper. The financial assistance of DST, New Delhi is appreciated.
References [1] S.M. Kuznicki, U.S. Patent 4,853 (1989) 202. [2] D.M. Chapman, A.L. Roe, Zeolite 10 (1990) 730. [3] S.M. Kuznicki, K.A. Trush, F.M. Allen, S.M. Levine, M.M. Hamil, D.T. Hayhurst, M. Mansour, in: M.L. Occelli, H.E. Robson (Eds.), Molecular Sieves, Synthesis of Microporous Materials, Vol. 1, Van Nostrand–Reinold, New York, 1992, p. 427. [4] R.G. Anthony, C.V. Philip, R.G. Dosch, Waste Manage. 13 (1993) 503. [5] A. Clearfield, J.A. Stynes, J. Inorg. Nucl. Chem. 26 (1964) 117. [6] A. Clearfield, R.H. Blessing, J.A. Stynes, J. Inorg. Nucl. Chem. 30 (1968) 2249. [7] B.B. Sahu, H.K. Mishra, K.M. Parida, J. Colloid Interface Sci. 225 (2000) 511–519. [8] G. Alberti, P. Cardini Galli, U. Costantino, E. Torracca, J. Inorg. Nucl. Chem. 29 (1967) 571. [9] K.M. Parida, P.K. Satapathy, A.K. Sahoo, N.N. Das, J. Colloid Interface Sci. 173 (1995) 122. [10] A. Clearfield, G.D. Smith, J. Inorg. Chem. 8 (1969) 431. [11] G. Alberti, E. Torracca, J. Inorg. Nucl. Chem. 30 (1968) 317. [12] F. Michel, A. Weiss, Z. Naturforsch. B 22 (1967) 1100. [13] R. Llavona, J.R. Garcia, M. Suarez, J. Rodriguez, Thermochim. Acta 86 (1985) 281. [14] C.B. Amphlett, Inorganic Ion Exchangers, Elsevier, Amsterdam, 1964. [15] A. Clearfield, G.D. Smith, J. Inorg. Chem. 8 (1969) 431. [16] A. La Ginestra, M.A. Massucci, Thermochim. Acta 32 (1979) 241. [17] J.M. Gray, M.A. Malati, J. Chem. Technol. Biotechnol. 29 (1979) 135. [18] D.D. Perrin, B. Dempsey, Buffer for pH Metal Ion Control, Chapman & Hall, London, 1974. [19] L. Maya, P.O. Danis, J. Chromatogr. 190 (1980) 145–149. [20] C.B. Amphlett, L.A. McDonald, M.J. Redman, J. Inorg. Nucl. Chem. 6 (1958) 220. [21] L. Kulberg, A. Clearfield, J. Phys. Chem. 89 (1985) 1585. [22] B. Rebenstorf, T. Linhald, S.L.T. Anderson, J. Catal. 128 (1991) 293. [23] A. Clearfield, W. L Duax, A. S Meduna, G.D. Smith, J.R. Thomas, J. Phys. Chem. 73 (1969) 3424. [24] S.J. Harvie, G.H. Nancollas, J. Inorg. Nucl. Chem. 32 (1970) 2923. [25] A. Lewandowski, W. Czemplik, A. Dzik, Mat. Fiz. Chem. 3 (1960) 3. [26] M.F. Smirnova, A.P. Doshina, V.B. Aleskovskii, Izv. Akad. Nauk SSSR Neorg. Mater. 4 (1968) 248. [27] A. Clearfield, A.I. Bortun, L.N. Bortun, Solv. Extract. Ion Exchange 16 (2) (1998) 669. [28] J. Albertsson, Acta Chem. Scand. 20 (6) (1966) 1689. [29] C.P. Huang, W. Sfumm, J. Colloid Interface Sci. 213 (1973) 409.
A comparative study on textural characterization: cation-exchange and sorption properties of crystalline α-zirconium(IV), tin(IV), and titanium(IV) phosphates K.M. Parida,∗ B.B. Sahu, and D.P. Das Regional Research Laboratory (CSIR), Bhubaneswar 751013, Orissa, India Received 20 May 2003; accepted 30 September 2003
Abstract Tetravalent metal phosphates (M = Zr, Ti, and Sn) were prepared and characterized by XRD, surface properties, and TG-DTA. The cation exchange and sorption behavior of these metal phosphates toward transition metal ions such as Cu2+ , Co2+ , and Ni2+ have been studied comparatively as a function of temperature and concentration. The adsorption process was found to increases with increase in temperature and concentration. The selectivity order for α-titanium and α-tin phosphates is Cu2+ > Co2+ > Ni2+ , whereas for α-zirconium phosphate it is Cu2+ > Ni2+ > Co2+ . The ion exchange capacity of α-titanium phosphate is greater than those of other phosphates, which is explained on the basis of the surface behavior, disorderness of the system, degree of hydrolysis of incoming guest adsorbate metal ions, and structural steric hindrance of the exchangers during adsorption and sorption. The distribution coefficient, Gibbs free energy, enthalpy, and entropy values indicate that the ion-exchange processes are spontaneous. 2003 Elsevier Inc. All rights reserved. Keywords: Tetravalent metal phosphates; Fibrous layered ion exchangers; Sorption behavior; Distribution coefficient; Gibbs free energy
1. Introduction An active search for new selective ion exchangers and adsorbents undertaken in the past several decades resulted in discovery of a wide variety of synthetic inorganic compounds exhibiting ion-exchange properties [1]. Because of their high thermal/chemical stability, resistance to oxidation and radiation, and greater affinity to certain ions, these compounds (hydrated oxides and insoluble acid salts of polyvalent metals, heteropolyacids, metal ferrocyanides, etc.) are regarded as promising materials for operating under extreme conditions such as high temperature, radiation, pressure, concentration of background electrolytes, and presence of oxidants and organic solvents [2]. The structure of the defined crystalline tetravalent metal phosphates is layered and exhibits ion-sieve and enhanced catalytic properties [3]. Besides possible application of these materials in chemical processing of radioactive materials, * Corresponding author.
E-mail address: [email protected] (K.M. Parida). 0021-9797/$ – see front matter 2003 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2003.09.045
treatment of contaminated moderator and cooling water in nuclear fields, new applications have recently been found in water desalination (hydrometallurgy) and in fuel cells employing ion-exchange membranes for transport of hydrogen ions and thin-layer chromatographic separation of Na+ from K+ (electrodialysis). Ion exchange is now a wellestablished technique in many industrial processes and is also widely employed in many chemical laboratories. Due to the presence of both Brønsted and Lewis acid sites on their surfaces, they have 102 –103 times greater ion-exchange capacity than commercial organic ion exchangers such as Dowex 50-X8. New developments in this field led to the synthesis of several exchangers having a fixed composition and a well-defined crystalline structure [4,5]. In the present work we have made a comparative study of the textural characterization and cation exchange cum sorption properties of α-zirconium, titanium, and tin phosphates toward alkali and divalent metal cations. The corrected selectivity coefficients of various parameters as a function of temperature, concentration, and metal loading were also analyzed.
K.M. Parida et al. / Journal of Colloid and Interface Science 270 (2004) 436–445
2. Experimental 2.1. Method of preparation and chemical analysis Crystalline zirconium(IV) phosphate [Zr(HPO4)2 ·H2 O, α-ZrP], tin(IV)phosphate [Sn(HPO4 )2 ·H2 O, α-SnP], and titanium(IV) phosphate [Ti(HPO4 )2 ·H2 O, α-TiP] were prepared and analyzed using the previous experimental procedures [6–8]. 2.2. Physicochemical characterizations X-ray diffraction studies of the samples calcined at 500 ◦ C were carried out using a Phillips PW 1710 diffractometer fitted with an automatic control and data acquisition system (PC-APD Software). The patterns were run with monochromatic CoKα radiation (α = 1.72 Å). The 2θ diffraction angle (3–70◦ ) was scanned at the rate of 2.4◦ min−1 . TG-DTA analysis of room temperature and 110 ◦ C dried samples were carried out in dry air using a Shimadzu DT 40 thermal analyzer in the range from 25 to 1000 ◦ C at a heating rate of 10 ◦ C min−1 . Surface acidity and basicity were determined by the spectrophotometric method [9] on the basis of irreversible adsorption of organic bases such as pyridine (PY, pKa = 5.3), piperidine (PP, pKa = 11.1), morpholine (MR, pKa = 16.6), 2,6-dimethyl piperidine (DMP, pKa = 11.11) and acidic substrates such as acrylic acid (AR, pKa = 4.2) and phenol (PH, pKa = 9.9), respectively. One electron donor and one electron acceptor properties were determined on the basis of irreversible adsorption of phenothiazine (PNTZ, ionization energy, I.E. 7.13 eV) and 1,3-dinitrobenzene (DNB, electron affinity, E.A. 1.26 eV). Potentiometric titration of materials in 0.1 mol/L (Na/K)Cl solution at different temperatures (300, 310, and 320 K) was carried out in a thermostatic double-wall Pyrex cell of capacity 100 ml with a rubber lid equipped with holes for the electrode and a microburette. In each of the experiments, 25 ml of electrolyte (Na/K)Cl and 0.1 g of exchanger were put into the cell with regular stirring until the pH become constant. After equilibrium, the pH of the suspension was measured by an Elico digital pH meter (L1 120, India) and then standard 0.01 mol/L (Na/K)OH solution was added from the microburette. The pH of the suspension was recorded when the pH drift was less than 0.01 unit/min. The addition of base was continued until the pH of the solution reached 9.0. The adsorption of first-row transition metal ions (Cu2+ , 2+ Ni , and Co2+ ) was carried out with 50 ml of the metal ion solution in a 100 ml stoppered flask. About 0.1 g of metal phosphate was added to the solution and the pH was measured. The resulting suspension was shaken mechanically with a shaker (Julabo) for 24 h. The solution was filtered and the equilibrium pH of the filtrate was measured. Preliminary experiments showed that 24, 30, and 48 h are required
437
to reach the equilibrium pH for α-SnP, α-ZrP, and α-TiP, respectively. The pH was maintained in the range 2.7–4.1 for α-ZrP, 2.8–4.2 for α-SnP, and 2.6–3.8 for α-TiP. The concentration of the metal ion in the filtrate was determined with an atomic absorption spectrophotometer (Perkin–Elmer). The adsorption experiments were repeated at four different concentrations (8, 6, 4, and 2) ×10−4 mol l−1 of the metal ions.
3. Results and discussion 3.1. Textural properties The powder XRD pattern of 383 K heated α-ZrP, αTiP, and α-SnP showed crystalline structures having basal spacings 7.56, 7.6, and 7.76 Å, respectively, which is quite similar to the data reported earlier [10–12]. The Na+ and K+ exchanged form of tetravalent metal phosphates seems to exhibit a higher degree of crystallinity. From the powdered XRD pattern of α-ZrP, α-TiP, and α-SnP calcined at 773 K (Fig. 1), it was observed that the zirconium phosphate is completely converted into zirconium pyrophosphate (ZrP2 O7 ) and the case of tin phosphate is similar to that of zirconium phosphate. But in the case of titanium phosphate, at 773 K it is partly converted into pyrophosphate. From the thermogravimetric analysis (graph not given), it is observed that the total weight loss of α-ZrP, α-TiP, and α-SnP at 1173 K is 14.6%, 14.2%, and 14.1% corresponding to loss of 2.44, 2.03, and 2.22 moles of water per mole
Fig. 1. The XRD powder pattern of 773 K calcined (a) α-ZrP, (b) α-TiP, and (c) α-SnP.
438
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of material, respectively. The DTA curve exhibited two endothermic peaks at 468 and 531 K for α-ZrP, 573 and 773 K for α-TiP. However, there is only one endothermic peak at 429 K for α-SnP. The exothermic peak at 843 K for α-ZrP indicated a phase transformation of this material from the layered to a nonlayered structure corresponding to the formation of α-ZrP2 O7 . An exothermic peak for α-TiP was detected at 1086 K, which represents the transformation of α-TiP to α-TiP2 O7 [13]. Since one mole of H2 O must be related to the phosphate condensation, there could be two possibilities for the dehydration of α-SnP, which can be represented as follows: SnO(H2PO4 )2 Sn(HPO4 )2 + H2 O↑ α-Sn(HPO4 )2 ·H2 O → α-SnP2 O7 + 2H2 O↑. However, in contrast to other crystalline exchanges of this series, the weight loss curve does not distinctly show the two dehydration steps, and this fact could be explained on the basis of the overlapping of the two processes. To corroborate this hypothesis, the pyrophosphate content of this sample, heated at 623 K, indicated that 31% of the phosphate groups were converted to pyrophosphate. It is confirmed that the condensation begins before the first water molecule is completely lost. Further it was found that the α-SnP sample heated at 623 K and then suspended in water for 5 days takes up only 0.65 mol of water per formula weight, which is in good agreement with the value calculated using the pyrophosphate content. The monolayer adsorption of all the materials was assumed to be the acidity or basicity of the samples corresponding to a specific pKa of organic substrate used. The adsorption of organic substrates follow the Langmuir adsorption isotherm. From the adsorption data it has been observed that the chemisorption of piperidine and of acrylic acid show highest values of acid sites and basic sites, respectively, for all the samples. The general sequence obtained in all the cases for acid sites is PP > MR > PY > DMP. Ta-
ble 1 shows that the concentration of acid and basic sites of the materials increases with an increase in calcination temperature from 383 to 573 K and thereafter decreases to a low value at 773 K. It is well known that addition of transition metal to the metal phosphates enhances the concentration of one electron acceptor (oxidizing) site and one electron donor (reducing) site on the surface. Among all samples, α-TiP showed maximum increase in acid, basic, oxidizing, and reducing sites as compared to other materials. The oxidizing and reducing properties determined by the adsorption of DNB (m-dinitrobenzene) and PNTZ (phenothiazine), respectively, showed very little increase in these values with increase in calcination temperature up to 573 K, but decreased to a very low value beyond 773 K. As a consequence of the loss of bonded –OH groups above 573 K the above surface chemical properties of all materials decreased to a low value at 773 K. The above surface chemical properties decreased to a low value due to the loss of Brønsted acid sites. 3.2. Potentiometric titration The potentiometric titration curves of α-ZrP, α-TiP, and α-SnP in 0.1 mol/L of (Na/K)Cl at 300 K are presented in Fig. 2. For each case with addition of 0.01 mol/L of (Na/K)OH the pH increases. For α-ZrP, it exhibits two plateaus corresponding to the replacement of two hydrogens per mole of α-ZrP. The isotherm for α-ZrP shows two inflections below 7.5 and afterward it appears to be complicated due to hydrolysis effects. From Fig. 2 it was found that TiP is less hydrolyzed than ZrP and SnP. In the case of α-SnP
Table 1 Surface acidity, basicity, one-electron-donor, and one-electron-acceptor properties of α-ZrP, α-TiP, and α-SnP calcined at various temperatures Sample Temperature (◦ C)
Acid sites (µmol g−1 ) PP
Basic sites (µmol g−1 )
MR PY DMP AR
Redox sites (µmol g−1 )
PH PNTZ DNB
α-TiP
110 300 500
325 310 161 357 332 173 318 282 83
82 105 63
166 136 11.30 178 162 12.62 160 109 9.86
8.82 9.96 8.63
α-ZrP
110 300 500
316 281 80 341 299 133 276 202 69
75 100 42
162 111 10.06 163 132 11.37 160 102 9.50
8.65 9.52 8.28
α-SnP
110 300 500
275 223 294 241 236 201
67 90 33
153 119 10.01 160 124 11.14 111 100 9.20
8.48 9.42 8.10
94 96 87
Fig. 2. Potentiometric titration of α-ZrP, α-TiP, and α-SnP in an aqueous solution of 0.1 mol l−1 MCl (M = K and Na) at room temperature.
K.M. Parida et al. / Journal of Colloid and Interface Science 270 (2004) 436–445
439
Fig. 3. Potentiometric titration of α-TiP in an aqueous solution of 0.1 mol l−1 KCl in the presence of 50 ppm Cu2+ , Co2+ , and Ni2+ solution at 300 K.
Fig. 4. Potentiometric titration of α-SnP in an aqueous solution of 0.1 mol l−1 KCl at different temperatures.
there is a series of irregular kinks. There is no end point and no break point. This is an indication of weak ion exchanger behavior. From Fig. 3, it can be seen that the crystalline α-TiP shows a sigmoidal curve, which means that the surface selectivity of the exchanger toward various cations decreases as they progressively replace the protons from the surface of the microcrystals. Up to pH 4.0 there is negligible hydrolysis and the result differs slightly in the pH range 4.0–7.0 for α-TiP. The layered crystal structure of [Ti (HPO4 )2 ·nH2 O] is built up by bonding together these sandwiches by van der Waals forces. In this arrangement each P atom in the lower sandwich lies along a perpendicular line drawn from the Ti atom of the upper sandwich. This arrangement creates zeolite type cavities that are interconnected by windows whose maximum diameter is around 2.64 Å. Both H-atoms can be exchanged with other cations so that the total ion exchange capacity is 7.76 meq g−1 [14], which is greater than for α-ZrP (6.64 meq g−1 ) and α-SnP (6.08 meq g−1 ) and 4–12 times those of clay minerals [15]. At pH 7, the Na+ uptake on α-TiP is 7.15 meq g−1 . At higher pH values increased hydrolysis is observed and the ion-exchange capacity decreases. This results in a higher degree of conversion at higher pH and a decrease in ion exchange capacity caused by hydrolysis. Potentiometric titration curves of α-SnP at different temperature (300, 310, and 320 K) are presented in Fig. 4. It indicates that the pH of the suspension increases with the addition of 0.01 mol/L of KOH, which is in contrast to other systems [13] where the pH remains constant with the ion uptake.
Interestingly, for all the exchangers, the curve shows that the pH of the solution shifts toward lower values with increased temperature. This indicates that H+ liberation is facilitated by an increase in temperature, which can be expressed in the reaction x−n nRH + Mx+ + nH+ (aq) → Rn M (aq)
(1)
where RH is the exchanger and Mx+ is the metal ion. This shows that, with an increase in temperature, more and more H+ ions are displaced by the K+ /Na+ ions. Hence sorption of the metal ions varies proportionally with temperature. It was found that for α-TiP, the prolonged branch of the titration curve above pH 9.2 (300 K), 9.0 (310 K), and 7.8 (320 K) may be explained by titration of less acid groups including ≡Ti–OH groups [16]. Fig. 3 shows that all the titration curves of α-TiP in 0.1 mol l−1 KCl with different metal ions are placed at a lower pH value as compared to the blank titration, which was also observed in case of α-SnP [7], α-ZrP [11], AlPO4 , and Al(OH)3 [12]. Thus the mechanism of metal ion uptake in this case is similar to that of adsorption on oxides/hydroxides, which can be described by Eq. (1). The number of replaceable protons responsible for the ion exchange reaction can be estimated from the difference between the blank run and each titration curve in the presence of the exchangers. Thus the excess amount of base consumed in the presence of an exchanger gives the extent of sorption of metal cations. Therefore Eq. (1) can be rewritten by applying the law of mass action as K = (R)n Mx−n [H+ ]n [RH]n [Mx+ ]
(2)
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where K is the equilibrium constant and the symbols in the square brackets represent the respective concentrations. In the region of low adsorption, [RH]n and [Mx+ ] can be treated as constants and [(Rn )Mx−n ] is equal to Cb /n, where Cb is the volume of 0.01 mol l−1 KOH/NaOH added. Thus Eq. (2) can be simplified to K = [Cb /n][H+ ]n
(3)
or log Cb = [log K]n + npH.
(4)
Using Eq. (4), log Cb vs pH can be plotted to determine log K and n (Fig. 5). Plots of log Cb as a function of pH for the potentiometric titration of K+ ions for α-SnP are presented in Fig. 5. The values of log K and n for the exchanger are presented in Table 2. Interestingly, the curves (Fig. 5) show two distinct lines, indicating the complex mechanism of K+ ion sorption on
Fig. 5. Plot of log Cb vs pH for 0.1 mol l−1 KCl solution at different temperatures.
Table 2 Values of log K and n calculated from Eq. (1) for K+ exchange on α-TiP, α-ZrP, and α-SnP Temperature (K)
log K
n
α-TiP
300 310 320
−7.59 −7.73 −7.98
1.00 1.03 1.05
α-ZrP
300 310 320
−7.60 −7.76 −7.94
1.00 1.01 1.03
α-SnP
300 310 320
−7.61 −7.78 −7.93
1.00 1.10 1.20
Sample
α-SnP. This complication in the plot clearly indicates the inadequacy of Eq. (4) with the simplifying assumption as well as the presence of different types of exchange sites on α-SnP. The result also shows that Eq. (4) is only applicable between pH 3.0 and 5.0, indicating the limitation of the simplification. Therefore the equation is more appropriate at a low adsorption density. The break points for all three lines are the same, which shows that the uptake of K+ ions is independent of temperature. This probably shows that the number of high-energy sites responsible for the uptake of K+ ions is independent of temperature (300–320 K). The value of n determined (Table 2) from the line within the low pH region is nearly equal to 1, which suggests the ion-exchange stoichiometry for alkali metal ions in the exchange as RH + M+ → RM + H+ .
(5)
The low value of n at 300 K point toward the mechanism of metal-ion uptake rather than ion exchange. Other mechanisms could be simple adsorption or partition of electrolyte between the phases. The zeolitic nature of the exchangers and the weak forces between layers provide a basis for explaining their sieving behavior and ready expansion of the lattice during ion exchange [10]. Both phosphate hydrogen ions (HPO4 ) are replaced by cations. One proton is exchanged at a relatively low pH for alkali metal cations and the interlayer distance remains unchanged. Thus at half the exchange capacity, all the cavities contain one univalent cation and can accommodate no more. The second proton exchanges at higher pH and the cations insert themselves between the layers. Thus the mechanism of exchange is thought to involve the diffusion of unhydrated or partially hydrated ions into the cavities, replacing the more acidic phosphate protons. This is followed by diffusion of water molecules into the crystal lattice and subsequent rehydration of the cations. Further exchange can then occur by diffusion of hydrated ions into the sorbent cavities [17]. The Na+ is exchanged as the hydrated ion and K+ as the unhydrated ion [18]. 3.3. Adsorption of heavy metal ions Adsorption studies on α-ZrP, α-TiP, and α-SnP, carried out using suspensions containing different concentrations of Cu2+ , Co2+ , and Ni2+ ions at different temperatures, are shown in Fig. 6. It can be seen that the metal phosphates take up these metal ions in appreciable quantities and the uptake generally increases with increased in temperature and concentration of the metal ions. Selectivity of these exchangers for transition metal ions is found in the order Cu2+ > Co2+ > Ni2+ for α-TiP and α-SnP, whereas Cu2+ > Ni2+ > Co2+ is for α-ZrP. The same selectivity sequence as for α-ZrP was noted with Fe(OH)3 [19], SiO2 [20], ThO2 [21], and TiO2 [7], and of α-SnP, α-TiP was noted with AlPO4 [22], FePO4 [23], SnO2 [24], and MnO2 [25]. The sequence order for α-TiP and α-SnP is
K.M. Parida et al. / Journal of Colloid and Interface Science 270 (2004) 436–445
441
where RH2 is the exchanger and RM is the exchanged form, and + Sn(HPO4 )2 ·H2 O + Cu2+ (aq) → SnCu(PO4 )2 ·H2 O + 2H(aq) .
The adsorption data were fitted to linearly transformed Langmuir adsorption isotherms as Ceq /X = (1/bXm ) + Ceq /Xm ,
Fig. 6. Adsorption isotherm of Cu2+ , Co2+ , and Ni2+ on α-ZrP, α-TiP, and α-SnP at 300 K.
based on the stability of hexa-aqua ions, whereas for α-ZrP, this order can be related neither to the stability of hexa-aqua ions nor to the stereochemistry adopted by M2+ ions in the solid, which is distorted octahedral. However, comparing the extent of sorption of metal ions, the exchanger prefers the cation with low ionic potential pKa and low pH of hydrolysis (pKh ), where Kh is the hydrolysis constant of the metal cation in solution. As Cu2+ has the lowest pKh (=8.0), the adsorption is highest in comparison to Co2+ (pKh = 8.9) and Ni2+ (pKh = 9.9) [26]. The ion exchange and sorption properties increase with the increase in degree of crystallinity, which varies reciprocally with the interlayer spacing and surface area of the respective ion exchangers. Due to the minimum interlayer spacing and surface area of α-TiP as compared to α-ZrP and α-SnP, it has maximum ion exchange capacity (7.76 meq g−1 ). From the structures of these materials, it is seen that the Ti–Ti, Zr–Zr, and Sn–Sn distances are 5.3, 5.0 and 4.8 Å, respectively [27]. Therefore, the cavities of α-TiP are larger with a smaller steric hindrance for the incoming metal ions than others. As a result, sorption media facilitate its greater ion-exchange behavior. Due to the maximum interlayer distance of α-TiP, it requires a low activation energy (Ea ) for ion-exchange kinetics. In order to determine 2+ the mechanism of this process, H+ release /Madsorption for all the metal ions is given in Table 3. It can be concluded that on the average, 2 mol of H+ ions are released per 1 mol of metal ion adsorbed. This type of observation was also reported for the adsorption of various bivalent and trivalent cations on an oxide/hydroxide surface [19]. Therefore the exchange reaction can be expressed as RH2 + M2+ → RM + 2H+ ,
(6)
(7)
where Ceq is the equilibrium concentration of adsorbate in solution (mol l−1 ), X is the amount of adsorbate per unit mass of adsorbent (mol g−1 ), Xm is the amount adsorbed to form a monolayer (mol g−1 ), and b is the binding constant. Plots of Ceq /X vs Ceq give straight lines for α-ZrP, α-TiP, and α-SnP for all the metals Cu2+ , Co2+ , and Ni2+ (Figs. 7a, 7b, 7c, respectively) at 300 K with a correlation coefficient greater than 0.97 (Table 4), which favors the applicability of the Langmuir equation between pH 4.0 and 5.0. The values of Xm and b as determined from the slopes and intercepts of the lines at different temperatures are presented in Table 4. In order to obtain more quantitative information on the selectivity of exchangers, the distribution coefficient (Kd , ml g−1 ) values of the metal ions at different concentrations have been determined. The marked selectivity toward ions seen as Cu2+ , Co2+ , and Ni2+ can be pointed out so that the exchangers could be employed for several separations of analytical interest. According to A. Clearfield [27], Kd is calculated according to the formula 0 c c V /m − Ceq )/Ceq Kd = (Ceq (8) 0 and C c are the where Kd = distribution coefficient, Ceq eq ionic concentration of the initial solution and the solution after equilibrium with adsorbent, respectively. V /m (ml g−1 ) is the volume to mass ratio of the ion exchanger. According to Jorgen Albertsson [28]
log Kd = npH + D
(9)
where D = log K + n log(C − nCMG )
(10)
and 0 c CMG = (Ceq − Ceq )/(V /m) which is the total concentration of metal ion in the solution at equilibrium; C is the total exchange capacity of metal ion. When CMG is sufficiently small (CMG ≪ C), i.e., all sorption sites are equivalent, D is independent of CMG . Second, if the load (CMG ) on the ion exchanger is very low, and the constant value of CH (the hydrogen ion concentration) is so high that the solution can be considered to have a medium of constant ionic strength, it is found that Kd is a rectilinearly (rapidly) decreasing function of CMG alone, which indicates that sorption sites are not equivalent. It is observed from Table 5 that the sorption capacity of the solid
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Table 3 Stoichiometry of metal ion exchange on α-ZrP, α-TiP, and α-SnP at different concentrations at 320 K Sample α-TiP
α-ZrP
α-SnP
Metal ion
Initial metal ion conc. (10−4 mol l−1 )
Equilibrium conc. (10−4 mol l−1 )
Metal ion adsorbed, X (10−4 mol l−1 )
H+ released (10−4 mol l−1 )
H+ /M2+
Cu2+
2 4 6 8
0.003 0.063 0.150 0.263
1.997 3.94 5.85 7.737
3.99 8.27 11.99 16.01
2.00 2.10 2.05 2.07
Co2+
2 4 6 8
0.265 1.200 2.200 3.000
1.735 2.800 3.800 5.000
3.47 5.96 7.63 10.25
2.00 2.13 2.01 2.05
Ni2+
2 4 6 8
0.401 1.424 3.000 4.520
1.599 2.588 3.000 3.48
3.19 5.20 6.21 7.27
2.00 2.01 2.07 2.09
Cu2+
2 4 6 8
0.016 0.080 0.141 0.240
1.984 3.920 5.859 7.760
3.96 7.95 12.18 16.60
2.00 2.03 2.08 2.14
Co2+
2 4 6 8
0.488 1.640 3.180 4.612
1.512 2.360 2.820 3.388
3.05 4.79 5.86 7.04
2.02 2.03 2.08 2.07
Ni2+
2 4 6 8
0.300 1.230 2.220 3.120
1.700 2.770 3.780 4.880
3.36 5.56 7.52 9.81
1.98 2.01 1.99 2.01
Cu2+
2 4 6 8
0.020 0.085 0.154 0.266
1.980 3.915 5.846 7.734
3.97 7.83 11.10 14.46
2.01 2.00 1.90 1.87
Co2+
2 4 6 8
0.305 1.471 2.389 3.267
1.695 2.529 3.611 4.733
3.55 5.03 7.14 9.32
2.10 1.99 1.98 1.97
Ni2+
2 4 6 8
0.489 1.654 3.199 4.719
1.511 2.346 2.801 3.281
3.17 4.69 5.57 6.33
2.10 2.00 1.99 1.93
is very high and gradually decreases with decreased metal ion concentration. It is also seen that the adsorption sites are not of equal strength and the numbers of sites are different for different temperatures. Thus it is expected that the solid surface is covered by a monolayer of metal ions and a maximum number of sorption sites are active for metal ion adsorption. But beyond monolayer coverage, the metal ions repel the incoming homolog. The formation of monolayer coverage depends on the process temperature. From the variation of values of Kd (distribution coefficient) (Table 5) the sorption sites are not equivalent. That means that if the surface of the adsorbent is energetically heterogeneous rather than homogeneous, each step of the potentiometric titration isotherm will be replaced by an assortment of steps, corresponding to the completion of a monolayer on
different homogeneous patches of the surface when an ion exchange process occurs with a phase transition, giving rise to an ionic form having a larger interlayer distance than the original one, the formation of a metastable supersaturated solid solution is possible in the reverse process. As a result the sorption sites are not equivalent. Due to the greater hydrolysis constant of Co2+ than Ni2+ , the former should have a higher ionizing capacity. But evidently Ni(II) is sorbed more strongly than Co(II) for α-ZrP due to a greater separation factor [Kd (Ni)/Kd (Co)], varying between 1.56 to 2.13, whereas this factor is less than 1 for α-TiP and α-SnP (0.46 to 0.60 for α-TiP and 0.47 to 0.55 for α-SnP). The log Kd vs pH functions are straight and approximately parallel lines, which, however, do not coincide even
K.M. Parida et al. / Journal of Colloid and Interface Science 270 (2004) 436–445
(a)
443
(b)
(c) Fig. 7. Langmuir plots for the adsorption of (a) Cu2+ , (b) Ni2+ , and (c) Co2+ on α-ZrP, α-TiP, and α-SnP at 300 K.
at the lower loads investigated. The slope of the lines reaches a theoretical value of 2 for divalent cations. The apparent equilibrium constant (Ka ) (Table 6) corresponding to the adsorption process was calculated [20] as the product of the Langmuir parameters b and Xm and can be used as relative indicators of the affinity of the adsorbents toward the metal ions. The data (Table 2) shows that the affinity of α-ZrP, α-TiP, and α-SnP toward metal ions increases with an increase in temperature. From the plots of ln b vs T −1 (Fig. 8), the standard enthalpy (H 0 ) and en-
tropy (S 0 ) changes were calculated using the equation ln b = S 0 /R − H 0 /RT .
(11)
From the laws of thermodynamics (Gibbs–Helmholtz equation), G0 = H 0 − T S 0 .
(12)
By using values of S 0 and H 0 in Eq. (12), the Gibbs free energy (G0 ) can be calculated. All the values S 0 , H 0 , and G0 are presented in Table 7. The positive value of
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Table 4 Langmuir parameter Xm (mol g−1 ) and binding constant b for metal ion exchange on α-TiP, α-ZrP, and α-SnP Cu2+
Sample Temperature (K)
104 Xm
α-TiP
300 310 320
α-ZrP
α-SnP
Co2+ b
104 Xm
7.213 7.219 7.747
106266 114004 120601
300 310 320
6.120 6.234 6.618
300 310 320
5.004 5.062 6.514
Ni2+
Table 6 Apparent equilibrium constant (Ka ) and correlation coefficient calculated from Langmuir equation Ka
Sample Temperature
Correlation coefficient
(K)
Cu2+ Co2+ Ni2+
α-TiP
300 310 320
7.665 2.630 0.542 0.9924 0.9962 8.230 4.460 0.666 0.9864 0.9866 9.343 7.933 0.740 0.9821 0.9821
0.9998 0.9845 0.9716
47619 57871 58219
α-ZrP
300 310 320
6.205 0.423 2.000 0.9921 0.9721 6.310 0.565 2.463 0.9823 0.9723 7.216 0.629 3.028 0.9723 0.9746
0.9923 0.9810 0.9723
7958 13259 18315
α-SnP
300 310 320
5.267 0.234 0.197 0.9795 0.9864 5.595 0.503 0.250 0.9753 0.9874 8.149 0.523 0.349 0.9815 0.9819
0.9883 0.9997 0.9954
b
104 Xm
b
5.353 5.520 6.006
49131 80797 132084
0.908 0.970 0.973
59691 68659 76053
101388 101219 109035
1.790 1.960 2.001
23631 28826 31434
4.200 4.256 5.201
105267 110533 125109
2.946 3.594 3.613
11109 14019 14484
1.780 1.890 1.910
Cu2+
Co2+
Ni2+
Table 5 Distribution coefficient (Kd ) of different metal ion exchanges on α-ZrP, α-TiP, and α-SnP at different concentrations and temperatures Sample Metal ion α-ZrP
α-TiP
α-SnP
Initial metal
300 K
310 K
320 K
ion conc. log Kd pH log Kd pH log Kd pH (10−4 mol l−1 )
Cu2+
2 4 6 8
4.79 4.38 4.31 4.20
3.4 3.1 2.9 2.8
4.69 4.38 4.32 4.26
3.4 3.1 2.9 2.8
4.69 4.38 4.32 4.22
3.4 3.1 2.9 2.7
Co2+
2 4 6 8
3.19 2.85 2.64 2.56
3.5 3.3 3.2 3.1
3.18 2.86 2.64 2.57
3.5 3.3 3.2 3.1
3.18 2.86 2.66 2.59
3.5 3.3 3.2 3.1
Ni2+
2 4 6 8
3.45 3.05 2.92 2.89
3.5 3.3 3.1 3.0
3.45 3.06 2.93 2.89
3.4 3.2 3.1 3.0
3.48 3.09 2.94 2.91
3.5 3.2 3.1 2.9
Cu2+
2 4 6 8
5.52 4.49 4.29 4.16
3.3 3.1 2.9 2.8
4.99 4.69 4.51 4.38
3.4 3.1 3.0 2.8
5.00 4.70 4.56 4.39
3.5 3.2 3.0 2.9
Co2+
2 4 6 8
3.51 3.06 2.93 2.92
3.4 3.2 3.1 2.9
3.50 3.17 3.04 2.96
3.4 3.2 3.0 2.9
3.52 3.18 3.04 2.97
3.4 3.2 3.0 2.9
Fig. 8. Plot of ln b vs T −1 for different metal ions.
Ni2+
2 4 6 8
3.29 2.95 2.69 2.58
3.4 3.2 3.2 3.1
3.30 3.19 3.10 3.06
3.5 3.2 3.0 2.9
3.32 3.19 3.11 3.08
3.5 3.2 3.1 2.9
Table 7 Entropy, enthalpy, and Gibbs free energy changes of metal ions exchanged on α-TiP, α-ZrP, and α-SnP as a function of temperature
2 4 6 8
4.69 4.36 4.27 4.16
3.4 3.1 2.9 2.8
4.76 4.74 4.55 4.46
3.4 3.1 2.9 2.8
4.57 4.55 4.34 4.20
3.4 3.1 2.9 2.8
2 4 6 8
3.44 2.93 2.87 2.85
3.4 3.3 3.1 3.0
3.64 3.45 3.20 3.10
3.4 3.1 3.0 2.9
3.68 3.38 3.22 3.09
3.4 3.2 3.0 2.9
2 4 6 8
3.18 2.85 2.64 2.54
3.5 3.3 3.2 3.2
3.26 2.97 2.69 2.52
3.5 3.3 3.2 3.2
3.25 2.88 2.71 2.55
3.5 3.3 3.2 3.2
Cu2+
Co2+
Ni2+
Sample Metal ion
S 0 H 0 (JK−1 mol−1 ) (KJ mol−1 )
G0 (KJ mol−1 ) 300 K
310 K
320 K
Co2+ Ni2+
204.33 153.46 111.20
57.91 48.16 19.84
−61.26 −63.28 −65.33 −45.99 −47.53 −49.06 −29.73 −30.73 −31.71
α-ZrP
Cu2+ Co2+ Ni2+
165.00 101.25 118.23
41.26 18.00 34.50
−30.26 −31.26 −34.00 −24.64 −25.23 −27.47 −27.00 −28.68 −29.12
α-SnP
Cu2+ Co2+ Ni2+
119.42 113.51 100.58
10.17 7.03 6.85
−28.79 −29.98 −31.18 −23.87 −25.01 −26.15 −23.31 −24.32 −25.33
α-TiP
Cu2+
K.M. Parida et al. / Journal of Colloid and Interface Science 270 (2004) 436–445
H 0 indicates the endothermic nature of the ion-exchange system. The negative value of G0 favors the adsorption process. S 0 is positive for all the experiments, indicating an increased disorderedness in the system. In comparison with the ions of the same valency, the enthalpy change decreases with the atomic number of the metal ion, corresponding to a decrease in the entropy change of dehydration in response to an enthalpy–entropy compensation relation for α-ZrP only. This relation has been found in a wide variety of processes and reaction equilibria, heterogeneous catalysis, diffusion in metals, ionic crystals, and so on [29]. In the ion exchange system, the relation implies that the contribution of the electrostatic enthalpy term to the free energy is more or less fully compensated for by a simultaneous change of entropy term related to the hydration of the metal ion and the ion-exchanger phase. Thus the sorption of water molecules is responsible for the entropy change. The positive values of S 0 indicate the partial dehydration of the metal ions before adsorption, thus increasing the spontaneity. Again both H 0 and S 0 values followed the same order (Cu2+ > Co2+ > Ni2+ ) as the selectivity shown by α-TiP and α-SnP, whereas for α-ZrP the order is Cu2+ > Ni2+ > Co2+ . From these results, it can be concluded that changes in the hydration of metal ion play a dominant role in determining the selectivity of the exchanger. The highest disorder of the system was for Cu2+ (Table 7) as a result of hydration. This is also supported by the highest value of H 0 in the series, which indicates that a higher energy is needed to dehydrate Cu2+ ions than other mentioned ions. The value of G0 becomes more negative with increasing temperature, which shows that the adsorption is favored by an increase in temperature. The free energy changes responsible for the metal ion with H+ exchange as a function of the radius of the incoming cation show a departure from a linear relationship. This behavior is interpreted in terms of increasing distortion of the ion-exchanger matrix with the size of the incoming ion.
4. Conclusions From the foregoing discussion, it is clear that crystalline α-zirconium(IV), titanium(IV), and tin(IV) phosphate behave as acidic ion exchangers and have appreciable sorption capacity toward transition metal ions. The selectivity of metal ions for titanium(IV) and tin(IV) phosphates is in the order Cu2+ > Co2+ > Ni2+ , whereas for zirconium(IV) phosphate it is Cu2+ > Ni2+ > Co2+ . In all cases the extent of adsorption increases with increased temperature and concentration. Appearance of a plateau for α-ZrP, irregular kinks for α-SnP, and a sigmoidal curve for α-TiP in the potentiometric titration curves justify the presence of several exchangeable adsorption sites with different pKa values of
445
metal ions. The present investigation also shows that the ion exchange capacity (α-TiP > α-ZrP > α-SnP) depends on the degree of crystallinity, steric hindrance, sorption medium of exchangers, degree of hydrolysis, ionic potential, crystalline radius and charge of metal ions, and various thermodynamic parameters involved in the system.
Acknowledgments The authors are thankful to Dr. Vibhuti N. Misra, Director, Regional Research Laboratory, Bhubaneswar for his permission to publish this paper. The financial assistance of DST, New Delhi is appreciated.
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